Split four stroke engine

ABSTRACT

An engine includes a crankshaft, rotating about a crankshaft axis of the engine. A power piston is slidably received within a first cylinder and is operatively connected to the crankshaft via first linkage system. The power piston reciprocates through a power stroke and an exhaust stroke of a four stroke cycle during a single rotation of the crankshaft. A compression piston is slidably received within a second cylinder and operatively connected to the crankshaft via second linkage system. The first and second linkage systems share no common mechanical link. The compression piston reciprocates through an intake stroke and a compression stroke of the same four stroke cycle during the same revolution of the crankshaft. The power piston leads the compression piston by a phase shift angle that is substantially equal to or greater than zero degrees and less than 30 degrees.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This patent application is a continuation application of U.S.application Ser. No. 10/139,981, filed May 7, 2002, entitled “SPLIT FOURSTROKE ENGINE”, herein incorporated by reference in its entirety.

FIELD OF THE INVENTION

[0002] The present invention relates to internal combustion engines.More specifically, the present invention relates to a four-stroke cycleinternal combustion engine having a pair of offset pistons in which onepiston of the pair is used for the intake and compression strokes andanother piston of the pair is used for the power and exhaust strokes,with each four stroke cycle being completed in one revolution of thecrankshaft.

BACKGROUND OF THE INVENTION

[0003] Internal combustion engines are any of a group of devices inwhich the reactants of combustion, e.g., oxidizer and fuel, and theproducts of combustion serve as the working fluids of the engine. Thebasic components of an internal combustion engine are well known in theart and include the engine block, cylinder head, cylinders, pistons,valves, crankshaft and camshaft. The cylinder heads, cylinders and topsof the pistons typically form combustion chambers into which fuel andoxidizer (e.g., air) is introduced and combustion takes place. Such anengine gains its energy from the heat released during the combustion ofthe non-reacted working fluids, e.g., the oxidizer-fuel mixture. Thisprocess occurs within the engine and is part of the thermodynamic cycleof the device. In all internal combustion engines, useful work isgenerated from the hot, gaseous products of combustion acting directlyon moving surfaces of the engine, such as the top or crown of a piston.Generally, reciprocating motion of the pistons is transferred to rotarymotion of a crankshaft via connecting rods.

[0004] Internal combustion (IC) engines can be categorized into sparkignition (SI) and compression ignition (CI) categories. SI engines, i.e.typical gasoline engines, use a spark to ignite the air-fuel mixture,while the heat of compression ignites the air fuel mixture in CIengines, i.e., typically diesel engines.

[0005] The most common internal-combustion engine is the four-strokecycle engine, a conception whose basic design has not changed for morethan 100 years old. This is because of its outstanding performance as aprime mover in the ground transportation industry. In a four-strokecycle engine, power is recovered from the combustion process in fourseparate piston movements (strokes) of a single piston. For purposesherein, a stroke is defined as a complete movement of a piston from atop dead center position to a bottom dead center position or vice versa.Accordingly, a four-stroke cycle engine is defined herein to be anengine which requires four complete strokes of one or more pistons forevery power stroke, i.e. for every stroke that delivers power to acrankshaft.

[0006] Referring to FIGS. 1-4, an exemplary embodiment of a prior artfour stroke cycle internal combustion engine is shown at 10. Forpurposes of comparison, the following four FIGS. 1-4 describe what willbe termed a prior art “standard engine” 10. As will be explained ingreater detail hereinafter, this standard engine 10 is an SI engine witha 4 inch diameter piston, a 4 inch stroke and an 8 to 1 compressionratio. The compression ratio is defined herein as the maximum volume ofa predetermined mass of an air-fuel mixture before a compression stroke,divided by the volume of the mass of the air-fuel mixture at the pointof ignition. For the standard engine, the compression ratio issubstantially the ratio of the volume in cylinder 14 when piston 16 isat bottom dead center to the volume in the cylinder 14 when the piston16 is at top dead center.

[0007] The engine 10 includes an engine block 12 having the cylinder 14extending therethrough. The cylinder 14 is sized to receive thereciprocating piston 16 therein. Attached to the top of the cylinder 14is the cylinder head 18, which includes an inlet valve 20 and an outletvalve 22. The cylinder head 18 cylinder 14 and top (or crown 24) of thepiston 16 form a combustion chamber 26. On the inlet stroke (FIG. 1), afuel air mixture is introduced into the combustion chamber 26 through anintake passage 28 and the inlet valve 20, wherein the mixture is ignitedvia spark plug 30. The products of combustion are later exhaustedthrough outlet valve 22 and outlet passage 32 on the exhaust stroke(FIG. 4). A connecting rod 34 is pivotally attached at its top distalend 36 to the piston 16. A crankshaft 38 includes a mechanical offsetportion called the crankshaft throw 40, which is pivotally attached tothe bottom distal end 42 of connecting rod 34. The mechanical linkage ofthe connecting rod 34 to the piston 16 and crankshaft throw 40 serves toconvert the reciprocating motion (as indicated by arrow 44) of thepiston 16 to the rotary motion (as indicated by arrow 46) of thecrankshaft 38. The crankshaft 38 is mechanically linked (not shown) toan inlet camshaft 48 and an outlet camshaft 50, which precisely controlthe opening and closing of the inlet valve 20 and outlet valve 22respectively.

[0008] The cylinder 14 has a centerline (piston-cylinder axis) 52, whichis also the centerline of reciprocation of the piston 16. The crankshaft38 has a center of rotation (crankshaft axis) 54. For purposes of thisspecification, the direction of rotation 46 of the crankshaft 38 will bein the clockwise direction as viewed by the reader into the plane of thepaper. The centerline 52 of the cylinder 14 passes directly through thecenter of rotation 54 of the crankshaft 38.

[0009] Referring to FIG. 1, with the inlet valve 20 open, the piston 16first descends (as indicated by the direction of arrow 44) on the intakestroke. A predetermined mass of an explosive mixture of fuel (gasolinevapor) and air is drawn into the combustion chamber 26 by the partialvacuum thus created. The piston continues to descend until it reachesits bottom dead center (BDC), the point at which the piston is farthestfrom the cylinder head 18.

[0010] Referring to FIG. 2, with both the inlet 20 and outlet 22 valvesclosed, the mixture is compressed as the piston 16 ascends (as indicatedby the direction of arrow 44) on the compression stroke. As the end ofthe stroke approaches top dead center (TDC), i.e., the point at whichthe piston 16 is closest to the cylinder head 18, the volume of themixture is compressed to one eighth of its initial volume (due to an 8to 1 compression ratio). The mixture is then ignited by an electricspark from spark plug 30.

[0011] Referring to FIG. 3, the power stroke follows with both valves 20and 22 still closed. The piston 16 is driven downward (as indicated byarrow 44) toward bottom dead center (BDC), due to the expansion of theburned gas pressing on the crown 24 of the piston 16. Since the sparkplug 30 is fired when the piston 16 is at or near TDC, i.e. at itsfiring position, the combustion pressure (indicated by arrow 56) exertedby the ignited gas on the piston 16 is at its maximum at this point.This pressure 56 is transmitted through the connecting rod 34 andresults in a tangential force or torque (as indicated by arrow 58) onthe crankshaft 38.

[0012] When the piston 16 is at ifs firing position, there is asignificant clearance distance 60 between the top of the cylinder 14 andthe crown 24 of the piston 16. Typically, the clearance distance isbetween 0.5 to 0.6 inches. For the standard engine 10 illustrated theclearance distance is substantially 0.571 inches. When the piston 16 isat its firing position conditions are optimal for ignition, i.e.,optimal firing conditions. For purposes of comparison, the firingconditions of this engine 10 exemplary embodiment are: 1) a 4 inchdiameter piston, 2) a clearance volume of 7.181 cubic inches, 3) apressure before ignition of approximately 270 pounds per square inchabsolute (psia), 4) a maximum combustion pressure after ignition ofapproximately 1200 psia and 5) operating at 1400 RPM.

[0013] This clearance distance 60 corresponds typically to the 8 to 1compression ratio. Typically, SI engines operate optimally with a fixedcompression ratio within a range of about 6.0 to 8.5, while thecompression ratios of CI engines typically range from about 10 to 16:The piston's 16 firing position is generally at or near TDC, andrepresents the optimum volume and pressure for the fuel-air mixture toignite. If the clearance distance 60 were made smaller, the pressurewould increase rapidly.

[0014] Referring to FIG. 4, during the exhaust stroke, the ascendingpiston 16 forces the spent products of combustion through the openoutlet (or exhaust) valve 22. The cycle then repeats itself. For thisprior art four stoke cycle engine 10, four stokes of each piston 16,i.e. inlet, compression, power and exhaust, and two revolutions of thecrankshaft 38 are required to complete a cycle, i.e. to provide onepower stroke. Problematically, the overall thermodynamic efficiency ofthe standard four stroke engine 10 is only about one third (⅓). That is⅓ of the work is delivered to the crankshaft, ⅓ is lost in waste heat,and ⅓ is lost out of the exhaust.

[0015] As illustrated in FIGS. 3 and 5, one of the primary reasons forthis low 20 efficiency is the fact that peak torque and peak combustionpressure are inherently locked out of phase. FIG. 3 shows the positionof the piston 16 at the beginning of a power stroke, when the piston 16is in its firing position at or near TDC. When the spark plug 30 fires,the ignited fuel exerts maximum combustion pressure 56 on the piston 16,which is transmitted through the connecting rod 34 to the crankshaftthrow 40 of crankshaft 38. However, in this position, the connecting rod34 and the crankshaft throw 40 are both nearly aligned with thecenterline 52 of the cylinder 14. Therefore, the torque 58 is almostperpendicular to the direction of force 56, and is at its minimum value.The crankshaft 38 must rely on momentum generated from an attachedflywheel (not shown) to rotate it past this position.

[0016] Referring to FIG. 5, as the ignited gas expands in the combustionchamber 26, the piston 16 descends and the combustion pressure 56decreases. However, as the crankshaft throw 40 rotates past thecenterline 52 and TDC, the resulting tangential force or torque 58begins to grow. The torque 58 reaches a maximum value when thecrankshaft throw 40 rotates approximately 30 degrees past the centerline52. Rotation beyond that point causes the pressure 56 to fall off somuch that the torque 58 begins to decrease again, until both pressure 56and torque 58 reach a minimum at BDC. Therefore, the point of maximumtorque 58 and the point of maximum combustion pressure 56 are inherentlylocked out of phase by approximately 30 degrees.

[0017] Referring to FIG. 6, this concept can be further illustrated.Here, a graph of tangential force or torque versus degrees of rotationfrom TDC to BDC is shown at 62 for the standard prior art engine 10.Additionally, a graph of combustion pressure versus degrees of rotationfrom TDC to BDC is shown at 64 for engine 10. The calculations for thegraphs 62 and 64 were based on the standard prior art engine 10 having afour inch stroke, a four inch diameter piston, and a maximum combustionpressure at ignition of about 1200 PSIA. As can be seen from the graphs,the point of maximum combustion pressure 66 occurs at approximately 0degrees from TDC and the point of maximum torque 68 occurs approximately30 degrees later when the pressure 64 has been reduced considerably.Both graphs 62 and 64 approach their minimum values at BDC, orsubstantially 180 degrees of rotation past TDC.

[0018] An alternative way of increasing the thermal dynamic efficiencyof a four stoke cycle engine is to increase the compression ratio of theengine. However, automotive manufactures have found that SI enginestypically operate optimally with a compression ratio within a range ofabout 6.0 to 8.5, while CI engines typically operate best within acompression ratio range of about 10 to 16. This is because as thecompression ratios of SI or CI engines increase substantially beyond theabove ranges, several other problems occur which outweigh the advantagesgained. For example, the engine must be made heavier and bulkier inorder to handle the greater pressures involved. Also problems ofpremature ignition begin to occur, especially in SI engines.

[0019] Many rather exotic early engine designs were patented. However,none were able to offer greater efficiencies or other significantadvantages, which would replace the standard engine 10 exemplifiedabove. Some of these early patents included: U.S. Pat. Nos. 848,029;939,376; 1,111,841; 1,248,250; 1,301,141; 1,392,359; 1,856,048;1,969,815; 2,091,410; 2,091,411; 2,091,412; 2,091,413; 2,269,948;3,895,614; British Patent No. 299,602; British Patent No. 721,025 andItalian Patent No. 505,576. In particular the U.S. Pat. No. 1,111,841 toKoenig disclosed a prior art split piston/cylinder design in which anintake and compression stroke was accomplished in a compression piston12/cylinder 11 combination, and a power and an exhaust stroke wasaccomplished in an engine piston 7/cylinder 8 combination. Each piston 7and 12 reciprocates along a piston cylinder axis which intersected thesingle crankshaft 5 (see FIG. 3 therein). A thermal chamber 24 connectsthe heads of the compression and engine cylinders, with one end beingopen to the engine cylinder and the other end having a valued dischargeport 19 communicating with the compressor cylinder. A water cooled heatexchanger 15 is disposed at the top of the compressor cylinder 11 tocool the air or air/fuel mixture as it is compressed. A set of spacedthermal plates 25 are disposed within the thermal chamber 24 to re-heatthe previously cooled compressed gas as it passes through.

[0020] It was thought that the engine would gain efficiency by making iteasier to compress the gas by cooling it. Thereafter, the gas wasre-heated in the thermal chamber in order to increase its pressure to apoint where efficient ignition could take place. Upon the exhauststroke, hot exhaust gases were passed back through the thermal chamberand out of an exhaust port 26 in an effort to re-heat the thermalchamber.

[0021] Unfortunately, transfer of gas in all prior art engines of asplit piston design always requires work, which reduces efficiency.Additionally, the added expansion from the thermal chamber to the enginecylinder of Koenig also reduced compression ratio. The standard engine10 requires no such transfer process and associated additional work.Moreover, the cooling and re-heating of the gas, back and forth throughthe thermal chamber did not provide enough of an advantage to overcomethe losses incurred during the gas transfer process. Therefore, theKoenig patent lost efficiency and compression ratio relative to thestandard engine 10.

[0022] For purposes herein, a crankshaft axis is defined as being offsetfrom the piston cylinder axis when the crankshaft axis and thepiston-cylinder axis do not intersect. The distance between the extendedcrankshaft axis and the extended piston-cylinder axis taken along a linedrawn perpendicular to the piston cylinder axis is defined as theoffset. Typically, offset pistons are connected to the crankshaft bywell-known connecting rods and crankshaft throws. However, one skilledin the art would recognize that offset pistons may be operativelyconnected to a crankshaft by several other mechanical linkages. Forexample, a first piston may be connected to a first crankshaft and asecond piston may be connected to a second crankshaft, and the twocrankshafts may be operatively connected together through a system ofgears. Alternatively, pivoted lever arms or other mechanical linkagesmay be used in conjunction with, or in lieu of, the connecting rods andcrankshaft throws to operatively connect the offset pistons to thecrankshaft.

[0023] Certain technology relating to reciprocating piston internalcombustion engines in which the crankshaft axis is offset from, i.e.,does not intersect with, the piston-cylinder axes is described in U.S.Pat. Nos. 810,347; 2,957,455; 2,974,541; 4,628,876; 4,945,866; and5,146,884; in Japan patent document 60-256,642; in Soviet Union patentdocument 1551-880-A; and in Japanese Society of Automotive Engineers(JSAE) Convention Proceedings, date 1996, issue 966, pages 129-132.According to descriptions contained in those publications, the variousengine geometries are motivated by various considerations, includingpower and torque improvements and friction and vibration reductions.Additionally, in-line, or straight engines in which the crankshaft axisis offset from the piston axes were used in early twentieth centuryracing engines.

[0024] However, all of the improvements gained were due to increasingthe torque angles on the power stroke only. Unfortunately, as will bediscussed in greater detail hereinafter, the greater the advantage anoffset was to the power stroke was also accompanied by an associatedincreasing disadvantage to the compression stroke. Therefore, the degreeof offset quickly becomes self limiting, wherein the advantages totorque, power, friction and vibration to the power stroke do not outweigh the disadvantages to the same functions on the compression stroke.Additionally, no advantages were taught or discussed regarding offsetsto optimize the compression stroke.

[0025] By way of example, a recent prior art attempt to increaseefficiency in a standard engine 10 type design through the use of anoffset is disclosed in U.S. Pat. No. 6,058,901 to Lee. Lee believes thatimproved efficiency will result by reducing the frictional forces of thepiston rings on the side walls over the full duration of two revolutionsof a four stroke cycle (see Lee, column 4, lines 1016). Lee attempts toaccomplish this by providing an offset cylinder, wherein the timing ofcombustion within each cylinder is controlled to cause maximumcombustion pressure to occur when an imaginary plane that contains botha respective connection axis of a respective connecting rod to therespective piston and a respective connection axis of the connecting rodto a respective throw of the crankshaft is substantially coincident withthe respective cylinder axis along which the piston reciprocates.

[0026] However, though the offset is an advantage during the powerstroke, it becomes a disadvantage during the compression stroke. Thatis, when the piston travels from bottom dead center to top dead centerduring the compression stroke, the offset piston-cylinder axis createsan angle between the crankshaft throw and connecting rod that reducesthe torque applied to the piston. Additionally, the side forcesresulting from the poor torque angles on the compression stroke actuallyincrease wear on the piston rings. Accordingly, a greater amount ofpower must be consumed in order to compress the gas to complete thecompression stroke as the offset increases. Therefore, the amount ofoffset is severely limited by its own disadvantages on the compressionside. Accordingly, large prior art offsets, i.e., offsets in which thecrankshaft must rotate at least 20 degrees past a pistons top deadcenter position before the piston can reach a firing position, have notbeen utilized, disclosed or taught. As a result, the relatively largeoffsets required to substantially align peak torque to peak combustionpressure cannot be accomplished with Lee's invention.

[0027] Variable Compression Ratio (VCR) engines are a class of prior artCI engines designed to take advantage of varying the compression ratioon an engine to increase efficiency. One such typical example isdisclosed in U.S. Pat. No. 4,955,328 to Sobotowski. Sobotowski describesan engine in which compression ratio is varied by altering the phaserelation between two pistons operating in cylinders interconnectedthrough a transfer port that lets the gas flow in both directions.

[0028] However, altering the phase relation to vary compression ratiosimpose design requirements on the engine that greatly increase itscomplexity and decrease its utility. For example, each piston of thepair of pistons must reciprocate through all four strokes of a completefour stroke cycle, and must be driven by a pair of crankshafts whichrotate through two full revolutions per four stroke cycle. Additionally,the linkages between the pair of crankshafts become very complex andheavy. Also the engine is limited by design to CI engines due to thehigher compression ratios involved.

[0029] Various other relatively recent specialized prior art engineshave also been designed in an attempt to increase engine efficiency. Onesuch engine is described in U.S. Pat. No. 5,546,897 to Bracken entitled“Internal Combustion Engine with Stroke Specialized Cylinders”. InBrackett, the engine is divided into a working section and a compressorsection. The compressor section delivers charged air to the workingsection, which utilizes a scotch yoke or conjugate drive motiontranslator design to enhance efficiency. The specialized engine can bedescribed as a horizontally opposed engine in which a pair of opposedpistons reciprocate in opposing directions within one cylinder block.

[0030] However, the compressor is designed essentially as a supercharger which delivers supercharged gas to the working section. Eachpiston in the working section must reciprocate through all four strokesof intake, compression, power and exhaust, as each crankshaft involvedmust complete two full revolutions per four-stroke cycle. Additionally,the design is complex, expensive and limited to very specialized CIengines.

[0031] Another specialized prior art design is described in U.S. Pat.No. 5,623,894 to Clarke entitled “Dual Compression and Dual ExpansionEngine”. Clarke essentially discloses a specialized two-stroke enginewhere opposing pistons are disposed in a single cylinder to perform apower stroke and a compression stroke. The single cylinder and thecrowns of the opposing pistons define a combustion chamber, which islocated in a reciprocating inner housing. Intake and exhaust of the gasinto and out of the combustion chamber is performed by specializedconical pistons, and the reciprocating inner housing.

[0032] However, the engine is a highly specialized two-stroke system inwhich the opposing pistons each perform a compression stroke and a powerstroke in the same cylinder. Additionally, the design is very complexrequiring dual crankshafts, four pistons and a reciprocating innerhousing to complete the single revolution two-stroke cycle. Also, theengine is limited to large CI engine applications.

[0033] Accordingly, there is a need for an improved four-stroke internalcombustion engine, which can enhance efficiency by more closely aligningthe torque and force curves generated during a power stroke withoutincreasing compression ratios substantially beyond normally accepteddesign limits.

SUMMARY OF THE INVENTION

[0034] The present invention offers advantages and alternatives over theprior art by providing a four-stroke cycle internal combustion enginehaving a pair of pistons in which one piston of the pair is used for theintake and compression strokes and another piston of the pair is usedfor the power and exhaust strokes, with each four stroke cycle beingcompleted in one revolution of the crankshaft. The engine enhancesefficiency by more closely aligning the torque and force curvesgenerated during a power stroke without increasing compression ratios.

[0035] These and other advantages are accomplished in an exemplaryembodiment of the invention by providing a four stroke cycle internalcombustion engine including a crankshaft rotating about a crankshaftaxis of the engine. A power piston is slidably received within a firstcylinder and operatively connected to the crankshaft such that the powerpiston reciprocates through a power stroke and an exhaust stroke of afour stroke cycle during a single rotation of the crankshaft. Acompression piston is slidably received within a second cylinder andoperatively connected to the crankshaft such that the compression pistonreciprocates through an intake stroke and a compression stroke of thesame four stroke cycle during the same rotation of the crankshaft. Apower piston-cylinder axis along which the power piston reciprocateswithin the first cylinder has an offset from the crankshaft axis suchthat the power piston-cylinder axis does not intersect the crankshaftaxis. A gas passage interconnects the first and second cylinders. Thegas passage includes an inlet valve and an outlet valve defining apressure chamber therebetween. The inlet valve permits substantially oneway flow of compressed gas from the second cylinder to the pressurechamber and the outlet valve permits substantially one way flow ofcompressed gas from the pressure chamber to the first cylinder. Theinlet valve and the outlet valve of the gas passage substantiallymaintain at least a predetermined firing condition gas pressure in thepressure chamber during the entire four stroke cycle.

[0036] In an alternative embodiment of the invention the engine includesa compression piston-cylinder axis along which the compression pistonreciprocates within the second cylinder. The compression piston-cylinderaxis has an offset from the crankshaft axis such that the compressionpiston-cylinder axis does not intersect the crankshaft axis.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037]FIG. 1 is a schematic diagram of a representative prior art fourstroke cycle engine, during the intake stoke;

[0038]FIG. 2 is a schematic diagram of the prior art engine of FIG. 1during the compression stoke;

[0039]FIG. 3 is a schematic diagram of the prior art engine of FIG. 1during the power stoke;

[0040]FIG. 4 is a schematic diagram of the prior art engine of FIG. 1during the exhaust stoke;

[0041]FIG. 5 is a schematic diagram of the prior art engine of FIG. 1when the piston is at the position of maximum torque;

[0042]FIG. 6, is a graphical representation of torque and combustionpressure of the prior art engine of FIG. 1;

[0043]FIG. 7 is a schematic diagram of an engine in accordance with thepresent invention during the exhaust and intake strokes;

[0044]FIG. 8 is a schematic diagram of the engine of FIG. 7 when thefirst piston has just reached top dead center (TDC) at the beginning ofa power stroke;

[0045]FIG. 9 is a schematic diagram of the engine of FIG. 7 when thefirst piston has reached its firing position;

[0046]FIG. 10, is a graphical representation of torque and combustionpressure of the engine of FIG. 7; and

[0047]FIG. 11 is a schematic diagram of an alternative embodiment of anengine in accordance with the present invention having unequal throwsand piston diameters.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0048] Referring to FIG. 7, an exemplary embodiment of a four strokeinternal combustion engine in accordance with the present invention isshown generally at 100. The engine 100 includes an engine block 102having a first cylinder 104 and a second cylinder 106 extendingtherethrough. A crankshaft 108 is journaled for rotation about acrankshaft axis 110 (extending perpendicular to the plane of the paper).

[0049] The engine block 102 is the main structural member of the engine100 and extends upward from the crankshaft 108 to the junction with thecylinder head 112. The engine block 102 serves as the structuralframework of the engine 100 and typically carries the mounting pad bywhich the engine is supported in the chassis (not shown). The engineblock 102 is generally a casting with appropriate machined surfaces andthreaded holes for attaching the cylinder head 112 and other units ofthe engine 100.

[0050] The cylinders 104 and 106 are openings, typically of generallycircular cross section, that extend through the upper portion of theengine block 102. Cylinders are defined herein as the chambers withinwhich pistons of an engine reciprocate, and do not have to be generallycircular in cross section, e.g., they may have a generally elliptical orhalf moon shape.

[0051] The internal walls of cylinders 104 and 106 are bored andpolished to form smooth, accurate bearing surfaces sized to receive afirst power piston 114, and a second compression piston 116respectively. The power piston 114 reciprocates along a firstpiston-cylinder axis 113, and the compression piston 116 reciprocatesalong a second piston-cylinder axis 115. The first and second cylinders104 and 106 are disposed in the engine 100 such that the first andsecond piston-cylinder axes 113 and 115 pass on opposing sides of thecrankshaft axis 110 without intersecting the crankshaft axis 110.

[0052] The pistons 114 and 116 are typically cup shaped cylindricalcastings of steel or aluminum alloy. The upper closed ends, i.e., tops,of the power and compression pistons 114 and 116 are the first andsecond crowns 118 and 120 respectively. The outer surfaces of thepistons 114,116 are generally machined to fit the cylinder bore closelyand are typically grooved to receive piston rings (not shown) that sealthe gap between the pistons and the cylinder walls.

[0053] First and second connecting rods 122 and 124 each include anangle bend 121 and 123 respectively. The connecting rods 122 and 124 arepivotally attached at their top distal ends 126 and 128 to the power andcompression pistons 114 and 116 respectively. The crankshaft 108includes a pair of mechanically offset portions called the first andsecond throws 130 and 132, which are pivotally attached to the bottomopposing distal ends 134 and 136 of the first and second connecting rods122 and 124 respectively. The mechanical linkages of the connecting rods122 and 124 to the pistons 114, 116 and crankshaft throws 130,132 serveto convert the reciprocating motion of the pistons (as indicated bydirectional arrow 138 for the power piston 114, and directional arrow140 for the compression piston 116) to the rotary motion (as indicatedby directional arrow 142) of the crankshaft 108. The first pistoncylinder axis 113 is offset such that it is disposed in the imaginaryhalf plane through which the first crankshaft throw 130 rotates from itstop dead center position to its bottom dead center position. The secondpiston cylinder axis 115 is offset in the opposing imaginary half plane.

[0054] Though this embodiment shows the first and second pistons 114 and116 connected directly to crankshaft 108 through connecting rods 122 and124 respectively, it is within the scope of this invention that othermeans may also be employed to operatively connect the pistons 114 and116 to the crankshaft 108. For example a second crankshaft may be usedto mechanically link the pistons 114 and 116 to the first crankshaft108.

[0055] The cylinder head 112 includes a gas passage 144 interconnectingthe first and second cylinders 104 and 106. The gas passage includes aninlet check valve 146 disposed in a distal end of the gas passage 144proximate the second cylinder 106. An outlet poppet valve 150 is alsodisposed in an opposing distal end of the gas passage 144 proximate thetop of the first cylinder 104. The inlet check valve 146 and outletpoppet valve 150 define a pressure chamber 148 there between. The inletvalve 146 permits the one way flow of compressed gas from the secondcylinder 106 to the pressure chamber 148. The outlet valve 150 permitsthe one way flow of compressed gas from the pressure chamber 148 to thefirst cylinder 104. Though check and poppet type valves are described asthe inlet and the outlet valves 146 and 150 respectively, any valvedesign appropriate for the application may be used instead, e.g., theinlet valve 146 may also be of the poppet type.

[0056] The cylinder head 112 also includes an intake valve 152 of thepoppet type disposed over the top of the second cylinder 106, and anexhaust valve 154 of the poppet type disposed over the top to the firstcylinder 104. Poppet valves 150,152 and 154 typically have a metal shaft156 with a disk 158 at one end fitted to block the valve opening. Theother end of the shafts 156 of poppet valves 150, 152 and 154 aremechanically linked to camshafts 160,162 and 164 respectively. Thecamshafts 160,162 and 164 are typically a round rod with generally ovalshaped lobes located inside the engine block 102 or in the cylinder head112.

[0057] The camshafts 160,162 and 164 are mechanically connected to thecrankshaft 108, typically through a gear wheel, belt or chain links (notshown). When the crankshaft 108 forces the camshafts 160, 162 and 164 toturn, the lobes on the camshafts 160, 162 and 164 cause the valves150,152 and 154 to open and close at precise moments in the engine'scycle.

[0058] The crown 120 of compression piston 116, the walls of secondcylinder 106 and the cylinder head 112 form a compression chamber 166for the second cylinder 106. The crown 118 of power piston 114, thewalls of first cylinder 104 and the cylinder head 112 form a separatecombustion chamber 168 for the first cylinder 104. A spark plug 170 isdisposed in the cylinder head 112 over the first cylinder 104 and iscontrolled by a control device (not shown) which precisely times theignition of the compressed air gas mixture in the combustion chamber168. Though this embodiment describes a spark ignition (SI) engine, oneskilled in the art would recognize that compression ignition (CI)engines are within the scope of this invention also.

[0059] During operation, the power piston 114 1eads the compressionpiston 116 by a phase shift angle 172, defined by the degrees ofrotation the crankshaft 108 must rotate after the power piston 114 hasreached its top dead center position in order for the compression piston116 to reach its respective top dead center position. Preferably thisphase shift is between 30 to 60 degrees. For this particular preferredembodiment, the phase shift is fixed substantially at 50 degrees.

[0060]FIG. 7 illustrates the power piston 114 when it has reached itsbottom dead center (BDC) position and has just started ascending (asindicated by arrow 138) into its exhaust stroke. Compression piston 116is lagging the power piston 114 by 50 degrees and is descending (arrow140) through its intake stroke. The inlet valve 156 is open to allow anexplosive mixture of fuel and air to be drawn into the compressionchamber 166. The exhaust valve 154 is also open allowing piston 114 toforce spent products of combustion out of the combustion chamber 168.

[0061] The check valve 146 and poppet valve 150 of the gas passage 144are closed to prevent the transfer of ignitable fuel and spentcombustion products between the two chambers 166 and 168. Additionallyduring the exhaust and intake strokes, the inlet check valve 146 andoutlet poppet valve 150 seal the pressure chamber 148 to substantiallymaintain the pressure of any gas trapped therein from the previouscompression and power strokes.

[0062] Referring to FIG. 8, the power piston 114 has reached its topdead center (TDC) position and is about to descend into its power stroke(indicated by arrow 138), while the compression piston 116 is ascendingthrough its compression stroke (indicated by arrow 140). At this point,inlet check valve 146, outlet valve 150, intake valve 152 and exhaustvalve 154 are all closed.

[0063] At TDC piston 114 has a clearance distance 178 between the crown118 of the piston 114 and the top of the cylinder 104. This clearancedistance 178 is very small by comparison to the clearance distance 60 ofstandard engine 10 (best seen in FIG. 3). This is because the powerstroke in engine 100 follows a low pressure exhaust stroke, while thepower stroke in standard engine 10 follows a high pressure compressionstroke. Therefore, in distinct contrast to the standard engine 10, thereis little penalty to engine 100 to reduce the clearance distance 178since there is no high pressure gas trapped between the crown 118 andthe top of the cylinder 114. Moreover, by reducing the clearancedistance 178, a more thoroughly flushing of nearly all exhaust productsis accomplished.

[0064] In order to substantially align the point of maximum torque withmaximum combustion pressure, the crankshaft 108 must be rotatedapproximately 40 degrees past its top dead center position when thepower piston 114 is in its optimal firing position. Additionally,similar considerations hold true on the compression piston 116, in orderto reduce the amount of torque and power consumed by the crankshaft 108during a compression stroke. Both of these considerations require thatthe offsets on the piston-cylinder axes be much larger than any previousprior art offsets, i.e., offsets in which the crankshaft must rotate atleast 20 degrees past a pistons top dead center position before thepiston can reach a firing position. These offsets are in fact so largethat a straight connecting rod linking the pistons 114 and 116 wouldinterfere with the lower distal end of the cylinders 104 and 106 duringa stroke.

[0065] Accordingly, the bend 121 in connecting rod 122 must be disposedintermediate its distal ends and have a magnitude such that theconnecting rod 122 clears the bottom distal end 174 of cylinder 104while the power piston 114 reciprocates through an entire stroke.Additionally, the bend 123 in connecting rod 124 must be disposedintermediate its distal ends and have a magnitude such that theconnecting rod 124 clears the bottom distal end 176 of cylinder 106while the compression piston 116 reciprocates through an entire stroke.

[0066] Referring to FIG. 9, the crankshaft 108 has rotated an additional40 degrees (as indicated by arrow 180) past the TDC position of powerpiston 114 to reach its firing position, and the compression piston 116is Just completing its compression stroke. During this 40 degrees ofrotation, the compressed gas within the second cylinder 116 reaches athreshold pressure which forces the check valve 146 to open, while cam162 is timed to also open outlet valve 150. Therefore, as the powerpiston 114 descends and the compression piston 116 ascends, asubstantially equal mass of compressed gas is transferred from thecompression chamber 166 of the second cylinder 106 to the combustionchamber 168 of the first cylinder 104. When the power piston 114 reachesits firing position, check valve 146 and outlet valve 150 close toprevent any further gas transfer through pressure chamber 148.Accordingly, the mass and pressure of the gas within the pressurechamber 148 remain relatively constant before and after the gas transfertakes place. In other words, the gas pressure within the pressurechamber 148 is maintained at least (at or above) a predetermined firingcondition pressure, e.g., approximately 270 psia, for the entire fourstroke cycle.

[0067] By the time the power piston 114 has descended to its firingposition from TDC, the clearance distance 178 has grown to substantiallyequal that of the clearance distance 60 of standard engine 10 (best seenin FIG. 3), i.e., 0.571. Additionally, the firing conditions aresubstantially the same as the firing conditions of the standard engine10, which are generally: 1) a 4 inch diameter piston, 2) a clearancevolume of 7.181 cubic inches, 3) a pressure before ignition ofapproximately 270 pounds per square inch absolute (psia), and 4) amaximum combustion pressure after ignition of approximately 1200 psia.Moreover, the angle of the first throw 130 of crankshaft 108 is in itsmaximum torque position, i.e., approximately 40 degrees past TDC.‘Therefore, spark plug 170 is timed to fire such that maximum combustionpressure occurs when the power piston 114 substantially reaches itsposition of maximum torque.

[0068] During the next 10 degrees of rotation 142 of the crankshaft 108,the compression piston 116 will pass through to its TDC position andthereafter start another intake stroke to begin the cycle over again.The compression piston 116 also has a very small clearance distance 182relative to the standard engine 10. This is possible because, as the gaspressure in the compression chamber 166 of the second cylinder 106reaches the pressure in the pressure chamber 148, the check valve 146 isforced open to allow gas to flow through. Therefore, very little highpressure gas is trapped at the top of the power piston 116 when itreaches its TDC position.

[0069] The compression ratio of engine 100 can be anything within therealm of SI or CI engines, but for this exemplary embodiment it issubstantially within the range of 6 to 8.5. As defined earlier, thecompression ratio is the maximum volume of a predetermined mass of anair-fuel mixture before a compression stroke, divided by the volume ofthe mass of the air-fuel mixture at the point of ignition. For theengine 100, the compression ratio is substantially the ratio of thedisplacement volume in second cylinder 106 when the compression piston116 travels from SDC to TDC to the volume in the first cylinder 104 whenthe power piston 114 is at its firing position.

[0070] In distinct contrast to the standard engine 10 where thecompression stroke and the power stroke are always performed in sequenceby the same piston, the power stroke is performed by the power piston114 only, and the compression stroke is performed by the compressionpiston 116 only. Therefore, the power piston 116 can be offset to alignmaximum combustion pressure with maximum torque applied to thecrankshaft 108 without incurring penalty for being out of alignment onthe compression stroke. Vice versa, the compression piston 114 can beoffset to align maximum compression pressure with maximum torque appliedfrom the crankshaft 108 without incurring penalty for being out ofalignment on the power stroke.

[0071] Referring to FIG. 10, this concept can be further illustrated.Here, a graph of tangential force or torque versus degrees of rotationfrom TDC for power piston 114 is shown at 184 for the engine 100.Additionally, a graph of combustion pressure versus degrees of rotationfrom TDC for power piston 114 is shown at 186 for engine 100. Thecalculations for the graphs 184 and 186 were based on the engine 100having firing conditions substantially equal to that of a standardengine. That is: 1) a 4 inch diameter piston, 2) a clearance volume of7.181 cubic inches, 3) a pressure before ignition of approximately 270pounds per square inch absolute (psia), 4) a maximum combustion pressureafter ignition of approximately 1200 psia and 5) substantially equalrevolutions per minute (RPM) of the crankshafts 108 and 38. In distinctcontrast with the graphs of FIG. 6 for the standard prior art engine 10,the point of maximum combustion pressure 188 is substantially alignedwith the point of maximum torque 190. This alignment of combustionpressure 186 with torque 184 results in a significant increase inefficiency. Moreover, the compression piston's 116 offset can also beoptimized to substantially align the maximum torque delivered to thecompression piston 116 from the crankshaft 108 with the maximumcompression pressure of the gas. The compression pistons 116 offsetreduces the amount of power exerted in order to complete a compressionstroke and further increases the overall efficiency of engine 100relative to the standard engine 10. With the combined power andcompression piston 114, and 116 offsets, the overall theoreticalefficiency of engine 100 can be increased by approximately 20 to 40percent relative to the standard engine.

[0072] Referring to FIG. 11, an alternative embodiment of a split fourstroke engine having unequal throws and unequal piston diameters isshown generally at 200. Because the compression and power strokes areperformed by separate pistons 114, 116, various enhancements can be madeto optimize the efficiency of each stroke without the associatedpenalties incurred when the strokes are performed by a single piston.For example, the compression piston diameter 204 can be made larger thanthe power piston diameter 202 to further increase the efficiency ofcompression.

[0073] Additionally, the radius 206 of the first throw 130 for the powerpiston 114 can be made larger than the radius 208 of the second throw132 for the compression piston 116 to further enhance the total torqueapplied to the crankshaft 108.

[0074] While preferred embodiments have been shown and described,various modifications and substitutions may be made thereto withoutdeparting from the spirit and scope of the invention. Accordingly, it isto be understood that the present invention has been described by way ofillustration and not limitation.

What is claimed is:
 1. An engine comprising: a crankshaft, rotatingabout a crankshaft axis of the engine; a power piston slidably receivedwithin a first cylinder and operatively connected to the crankshaft viafirst linkage system such that the power piston reciprocates through apower stroke and an exhaust stroke of a four stroke cycle during asingle rotation of the crankshaft; a compression piston slidablyreceived within a second cylinder and operatively connected to thecrankshaft via second linkage system, wherein the first and secondlinkage systems share no common mechanical link, and such that thecompression piston reciprocates through an intake stroke and acompression stroke of the same four stroke cycle during the samerevolution of the crankshaft; wherein the power piston leads thecompression piston by a phase shift angle that is substantially equal toor greater than zero degrees and less than 30 degrees.
 2. The engine ofclaim 1 wherein the power piston leads the compression piston by a phaseshift angle that is substantially equal to or greater than 10 degreesand is substantially equal to or less than 29 degrees.
 3. An enginecomprising: a crankshaft, rotating about a crankshaft axis of theengine; a power piston slidably received within a first cylinder andoperatively connected to the crankshaft such that the power pistonreciprocates through a power stroke and an exhaust stroke of a fourstroke cycle during a single rotation of the crankshaft; a compressionpiston slidably received within a second cylinder and operativelyconnected to the crankshaft such that the compression pistonreciprocates through an intake stroke and a compression stroke of thesame four stroke cycle during the same rotation of the crankshaft;wherein the power piston leads the compression piston by a phase shiftangle that is substantially equal to or greater than 20 degrees and lessthan 30 degrees.
 4. The engine of claim 3 wherein the power piston leadsthe compression piston by a phase shift angle that is substantiallyequal to or greater than 20 degrees and is substantially equal to orless than 29 degrees.